The present invention generally relates to an air-fuel ratio control system for adjusting the air-fuel mixing ratio of a combustible air-fuel mixture, formed for an automobile internal combustion engine, to a predetermined or desired value and, more particularly, to a closed-loop air-fuel ratio control system which is effective to carry out the adjustment of the air-fuel mixing ratio according to a feed-back control scheme in dependence on the concentration of a selected component of exhaust gases emitted from the engine.
The control system of the type referred to above is shown and described in numerous patent publications and is, therefore, well known to those skilled in the art. Briefly speaking the control system comprises an electric circuit including a composition sensor for detecting the concentration of a component, usually oxygen, contained in the exhaust gases, and for generating a composition signal indicative of the detected concentration, a first controller which is operable in response to the composition signal to generate a ratio signal indicative of an optimum air-fuel mixing ratio based on the concentration of the detected component in the exhaust gases, a second controller which is operable in response to the ratio signal to generate a control signal, a parameter of which control signal is a function of the ratio signal, and an actuator electrically which is connected to the second controller to optimally control the air-fuel mixing ratio of the combustible mixture to be fed to the engine in dependence on the parameter of the control signal. The control of the air-fuel mixing ratio in dependence on the concentration of the exhaust gas component is possible in view of the fact that the concentration of the exhaust gas component is a function of the air-fuel mixing ratio of the combustible mixture which has been prepared in a carburetor and has subsequently burned in the engine.
The actuator which has been employed in the well known control system and referred to above is usually an electromagnetic valve provided in an air passage connected at one end thereof to an air bleed chamber in a carburetor or to another air passage bypassing the intake passage wherein a carburetor throttle valve is disposed. In either case, the air-fuel mixing ratio of the combustible mixture being supplied to the engine is optimized by selectively initiating and interrupting the supply of an additional air necessary respectively to lean and enrich the combustible mixture according to engine operating conditions.
In this well known control system, since the preparation of the combustible mixture of a definite air-fuel mixing ratio and the detection of the concentration of the particular component of the exhaust gases formed as a result of combustion of such combustible mixture of the definite air-fuel mixing ratio takes place at different times, a delay is inevitably involved, which delay brings about hunting of the signals being processed through the electric circuitry. The consequence is that the combustible mixture to be supplied to the engine tends to be alternately enriched and leaned relative to a predetermined air-fuel mixing ratio, the cycle of the alternation being dependent on the magnitude of the delay referred to above. Specifically, where the control gain in the electric circuitry is fixed, the larger the delay, the larger the deviation of the air-fuel mixing ratio from the predetermined optimum value, that is, the larger the hunting of the signals being processed in the circuitry.
Thus, with the well known control system described above, even during a normal operating condition of the engine wherein the air-fuel mixing ratio does not greatly deviate from the predetermined value, the delay in the system brings about hunting, resulting in a difficulty in maintaining the optimum air-fuel mixing ratio.
In order to substantially eliminate the above described problem, two techniques have been suggested. One technique is disclosed, for example, in Japanese Laid-open Patent Publication No. 51-124739 which was laid open to public inspection on Oct. 30, 1976. This publication discloses the employment of an idling sensor, an output signal of which is used to vary the control gain of the system to minimize the hunting which would occur during an idling of the engine. The idling sensor used therein is described as operable to detect one parameter or a combination of parameters representative of the idling of the engine, which parameters include the opening of the carburetor throttle valve, the engine speed, the position of an automobile transmission and the negative pressure inside the intake manifold.
However, minimization of the hunting achieved by the system of the above mentioned publication is still far from eliminating the previously discussed problem.
The other technique is disclosed, for example, in Japanese Laid-open Patent Publication No. 51-149420 which was laid open to public inspection on Dec. 22, 1976. This publication discloses the use of a circuit means including an idling sensor for generating a command signal in response to the detection of the idling of the engine, which command signal is used to disable the control system when the idling of the engine continues for more than a predetermined time and/or when the automobile starts running after the idling of the engine has continued for more than the predetermined time. As is the case with that in the first mentioned publication, the idling sensor used herein is operable to detect one parameter or a combination of the parameters representative of the engine idling condition.
Although the system disclosed in the second mentioned publication is effective to eliminate the hunting, optimization of the air-fuel mixing ratio of the combustible mixture to be supplied to the engine is sacrificed during the period in which the control system is disabled.